In an X-ray computed tomography apparatus (hereinafter referred to as the CT apparatus) having a narrow cone angle, projection data regarding the total of the heart cannot be sufficiently collected by scanning it as much as one rotation of an X-ray tube. To solve the above problem, helical scan is carried out at a slow helical pitch. Thus, in a CT apparatus having a wide cone angle, there is a case where the projection data regarding the total of the heart can be collected by the scan of one rotation of the X-ray tube. In this case, the projection data can be collected by volume scan without any movement of a bed.
Now, in examination of the heart in which the CT apparatus is used, it is one of important themes to improve a temporal resolution of an image. As a major method of dealing with this theme, there is a so called half EGR method in which both of a half reconstruction method and an electrocardiograph-gated reconstruction (EGR) method are used. As is well known, in this method, there is cut out a half projection data set which is collected during a time period for rotating an X-ray tube in an angle range of 180°+ a fan angle round a heartbeat phase designated by an operator. Further, an image is reconstructed from the half projection data set. The above heartbeat phase means that a time period from an R wave to the next R wave is standardized in a range of 0 to 100%, and a time point in the time period is expressed with a value of percentage.
According to the above method, the improvement of the temporal resolution can be achieved in contrast to a case of the scan over 360°. However, a time required for rotating the X-ray tube in an angle range of 180°+ the fan angle is restricted as a temporal resolution. Therefore, it is inevitable to deteriorate an image quality by blur or the like due to a change in a mode of the heart with the heartbeat which is brought about during a time period required for rotating the X-ray tube in an angle range of 180°+ the fan angle.
A segment EGR method is known as an image reconstructing method having the higher temporal resolution than in the case of the half EGR method. In the segment EGR method, projection data acquired in respective projection directions within an angle range of 180°+ the fan angle and at a timing near to a specific heartbeat phase are selected from a number of projection data collected during time periods corresponding to plural heartbeats, thereby obtaining the half projection data set. That is, the half projection data set is obtained by collecting the projection data acquired in substantially the same heartbeat phase from the projection data collected at respective different heartbeat periods. Consequently, time differences among timings for acquiring the respective projection data included in the half projection data set are increased, but time shifts based on a specific heartbeat phase are reduced, and hence the substantial temporal resolution is improved.
It is to be noted that a document that discloses a relevant technique is Jpn. Pat. Appin. KOKAI Publication No. 2007-037782.
However, the moving state of the heart may considerably vary, and there is no guarantee that the forms of the heart in the same heartbeat phase under the above definition are the same at two different points of time.
Therefore, in the segment EGR method, when there is no reproducibility of the motion of the heart during a plurality of heartbeat cycles in which a plurality of pieces of projection data included in a half projection data set are acquired, there is a fear that blurring occurs in a reconstructed image.
In particular, in the case of volume scan, it is difficult to smoothly mutually couple the projection data of a plurality of heartbeat cycles because of no movement of a bed, and hence the blurring in a reconstructed image is more remarkable than in the case of the helical scan.
Under such circumstances, it has been demanded to reconstruct an image with the high temporal resolution and the less blurring by the segment EGR method.